The present invention relates to a liquid phase oxidation catalyst and its preparation, and in particular to an oxidation catalyst especially suitable for oxidatively treating waste liquid having reducing ability and the like.
A pipe cleaner containing such a reductant as hydrogen peroxide and hydrazino is employed in various facilities for the purpose of removing scale accumulated on a boiler and a cooling water pipe. The cleaner is discharged as waste liquid after the pipe washing. When, however, due to the increase of chemical oxygen demand (COD), the waste liquid when a large amount of it is employed is required to be subject to such a treatment which makes the waste liquid harmless as an oxidation treatment with sodium hypochlorite.
Since the oxidation treatment with chemicals is expensive due to the necessity of the chemicals and facilities of the oxidation treatment, the treatment employing an oxidation catalyst comprising a metal support such as stainless steel and such a precious metal as palladium and platinum supported on the metal support has been of practical use.
The catalyst employed for the above application is conventionally prepared according to a baking process in which material such as palladium chloride which forms a catalyst layer and glassy material such as alumina which acts as a binder are applied on a metal substrate such as stainless steel and, after the sintering thereof, the substrate is baked to prepare the catalyst.
Many restrictions have been pointed out in the above process such that the evolution of such a harmful gas as hydrogen chloride and chlorine during the baking may occur and that the catalytic performance is likely to be influenced by the conditions for preparation such as a baking temperature and a blend ratio. In order to elevate the catalytic performance, the above restrictions must be resolved by means of the change of composition or the like.
The present inventors formerly attempted to provide the further catalytic performance by means of plating and invented an oxidation catalyst having a coating of platinum and, molybdenum and/or tungsten excellent in performance. However, the platinum was considerably expensive.
Then, the inventors further tried to develop another catalyst employing such less expensive material as palladium and having similar catalytic performance and a similar life to those of the platinum catalyst depending on the conditions employed. Although the catalytic performance similar to or higher than that of the catalyst was obtained by the baking process by means of palladium black plating, the adherence strength between the palladium layer and the metal substrate was inferior to produce the peel-off of the catalyst layer from the metal substrate so that the catalyst could not endure in the practical use. By means of ordinary palladium plating, the satisfactory adherence could be obtained but the sufficient catalytic performance could not be obtained.
The waste liquid after the pipe washing may contain another hydrochloric acid-based pipe cleaner as well as the above pipe cleaner containing the hydrogen peroxide and hydrazine so that the sufficient resistance to a running operation is also required.